The story
An allusion to an utterance of Jesus in John 8:7, viz. “He that is without sin among you, let him first cast a stone at her.”
Etymology from Wiktionary, CC BY-SA 4.0.
let him who is without sin cast the first stone
Today it means: Only those who are faultless have the right to pass judgment upon others (implying that no one is faultless and that, therefore, no one has such a right to pass judgment). Full definition →